A method to achieve allograft tolerance routinely in the clinic has not been realized. Most maneuvers creating a state of graft tolerance in preclinical models rely on a period of broad immunosuppressive therapy. As a result, the potential for creating a state of tolerance to any antigen (Ag) including an undesirable state of tolerance to microbial Ags exists, thereby heightening the risk of opportunistic infection or malignant transformation of EBV infected B-cells. In an ideal situation, graft tolerance would be achieved without reliance upon systemic application of broadly immunosuppressive agents. We are attempting to approach this goal by using strategies derived from empiric experiments in transplant models and modern insights into T cell immunobiology. "When cells of the immune system "see" antigens in the absence of the right cosignals, they shut themselves down instead of attacking. Future therapies might capitalize on that reaction" (1). Accordingly, we hypothesize that immunization of allograft recipients with manipulated donor organ grafts and/or leukocytes expressing histocompatibility Ags, but lack the capacity to directly deliver "co-signals" for Ag- stimulated T-cell activation, will cause a state of donor specific graft tolerance and bypass or vastly reduce the need for immunosuppressive treatment. The simple strategy being tested, if proven successful, can be rapidly deployed in the clinic. AIM 1: To test the hypothesis that transplantation of an allograft which has been manipulated so that it cannot instigate CD28/CTLA-4-pathway and/or CD2-pathway costimulatory T cell signals will create a state of donor specific graft tolerance in vivo. AIM 2: To test the hypothesis that immunization of allograft recipients with donor leukocyte preparations devoid of cells that express the proteins binding to immobilized CTLA-4 proteins and thereby cannot instigate CD28/CTLA-4- costimulatory signals will create a state of donor specific graft tolerance in vivo. AIM 3: To test the hypothesis that immunization with BOTH manipulated grafts (see AIM 1) an leukocyte preparations (see AIM 2) that cannot instigate costimulatory signals will prove to be superior to either maneuver alone in producing a state of donor specific graft tolerance in vivo with/without CsA therapy.